Developments of a light source apparatus using a laser beam, which is employed as a light source of a projection type projector used at a movie theater, for home theater equipment and the like, are proceeding. One type of the known laser light sources that are employed for such applications uses light directly emitted from a semiconductor laser element, and another type of the known laser light sources converts the light emitted from the semiconductor laser element to light having a different wavelength by a nonlinear optical crystal and uses the resulting light.
Recently, a blue laser light source and a green laser light source are developed, which use PPLN (Periodically Poled Lithium Niobate), PPLT (Periodically Poled Lithium Tantalate) and the like as the nonlinear optical crystal.
Such laser light sources are disclosed in, for example, Patent Literature 1 (Japanese Patent Application Laid-Open Publication No. 2009-54446). Patent Literature 1 describes a laser light apparatus that includes a light source having a semiconductor laser, a wavelength conversion element (i.e., nonlinear optical crystal such as PPLN) to receive a laser beam emitted from the light source and convert it to a laser beam having a second high harmonic wave, and an external resonator (e.g., VBG (Volume Bragg Grating) for selecting light having a predetermined wavelength from the light emitted from the wavelength conversion element and reflecting it toward the light source. Patent Literature 1 also discloses provision of a temperature adjusting unit between the laser light source apparatus and a subspace, in which the wavelength conversion element is mounted. In addition, Patent Literature 1 teaches an improvement in the light conversion efficiency by adjusting the temperature of the wavelength conversion element with the temperature adjusting unit that can adjust the pitch of the polarization reversal period of the wavelength conversion element.
FIG. 18 of the accompanying drawings is a block diagram showing an example of conventional laser light source apparatuses. Referring to FIG. 18, a conventional technology for setting the temperature of the wavelength conversion element to an optimal temperature will be described.
A wavelength conversion element (e.g., PPLN) 5 implemented on a laser light source unit LH has a wavelength conversion function of converting (shortening) the wavelength of light emitted from a laser light source element (e.g., semiconductor laser) 2 to a wavelength shorter than the incident light. For example, the wavelength conversion element 5 may convert infrared light to green light. The laser light source element is referred to as “semiconductor laser” hereinafter.
The wavelength conversion element (e.g., PPLN) 5 has a temperature (optimal temperature) that maximizes the light conversion efficiency. This optimal temperature is different from one element to another element, and the conversion efficiency decreases 10% or more if the temperature of the wavelength conversion element is deviated from the optimal temperature by 0.5 degrees C. Non-converted light simply becomes heat and is consumed.
Therefore, if the optimal temperature condition of the wavelength conversion element is sought and found, generally the following control is performed: the temperature detecting unit Th1 is used to detect the temperature of the wavelength conversion element 5, and the temperature adjusting unit is used to control the heating unit (e.g., heater 7) for heating the wavelength conversion element 5 from the outside such that the temperature of the wavelength conversion element 5 becomes the optimal temperature.
In order to find the optimal control temperature for the wavelength conversion element 5, the conventional technology employs the following approach.
The laser light source unit LH is driven by the laser light source power supply device 100, and the control temperature of the wavelength conversion element 5 is swept (scanned) in an assumed temperature range. In the meantime, the light output measuring device 110 (e.g., photocell, optical power meter or the like) is used to measure the light output emitted from the laser light source unit, as shown in FIG. 18 for example. When the sweeping (scanning) and measurement is finished, that control temperature which had the maximum light output is stored, and this value is taken as the optimal control temperature. The temperature of the wavelength conversion element is then controlled to become the optimal control temperature.
This approach, however, requires a unit for measuring the light output, and needs an additional cost associated with such unit.
FIG. 19 of the accompanying drawings shows a block diagram of another exemplary configuration of the conventional laser light source apparatuses. In this example, the lasers are connected in series.
As illustrated in FIG. 19, it is widely known to arrange a plurality of semiconductor lasers 2 in series inside a single laser light source power supply device 100 for the purpose of reducing the cost of the laser light source apparatus.
In this configuration, the switching elements and the control circuits in the laser light source power supply device 100 may have the same design, and therefore the source for power supply may be provided inexpensively. Three beams of light emitted from the respective laser light source units LH1-LH3 are concentrated (condensed) by optical elements such as prisms PZ, and the concentrated light is emitted.
When a plurality of laser elements are used in this manner, the respective wavelength conversion elements 5 require the optimal temperature adjustment individually. Thus, there is a problem that the optimal temperature adjustment needs significant time.
Specifically, when the measurement is carried out with the photocell as shown in FIG. 18, the sole laser light source unit LH is only measured, and the optimal temperature of this laser light source unit is adjusted. On the other hand, when the laser light source units LH1-LH3 are connected in series as shown in FIG. 19, the same current flows in the three laser elements, and therefore it is not possible to stop one of the three laser elements. As such, the optimal temperature adjustment of the wavelength conversion elements 5 is difficult.